An embodiment of an illumination device of the kind set forth is known from application WO2006054199. That document discloses a light source comprising a light engine, especially with at least one semiconductor light-emitting element like an LED and/or a Laser-Diode, and a light-guide as a primary optical system, which is retrofit so that it can be used in combination with a conventional secondary optical system like a reflector and/or a lens which is designed especially for the above mentioned conventional light sources and can substitute these without substantially degrading the radiation pattern characteristics.
The advantage of this approach lies in the fact that known secondary optical systems can be applied in combination with the enhanced performance of LEDs relative to conventional light sources, such as longer lifetime and lower energy consumption. Furthermore, the complete device can be fabricated in a conventional form factor, e.g. a low-voltage halogen PAR or dichroic reflector lamp.
However, one characteristic of the final light radiation pattern not covered by WO2006054199 is the color homogeneity in case a multitude of light-emitting elements are used, each with a different color. Color homogeneity is quite important for high quality lighting because the human eye is very sensitive for color differences between two points at close range. The problem especially exists when utilizing a multi-color array of LED chips for producing white light in a device with beam generating optics. An example of such a device would be a retrofit for a white light emitting halogen-based illumination device—such as a 50 mm diameter dichroic parabolic reflector lamp with a GU5.3 base—in which the array comprises a red, green and blue LED. It is then quite evident that the origin of the red, green and blue light is spatially separated. This spatial separation then becomes evident in the radiation pattern of the device if special precautions to mix these different colors are not taken.
Several solutions have been proposed to solve the mixing problem, but they all have the disadvantage that one or more criteria are sacrificed, such as: the size of the luminary, the angular width of the beam or the overall efficiency. Furthermore, some mixing solutions have very stringent alignment requirements.
One such well-known solution for the mixing problem is a light-guide. Such a ‘mixing rod’ has the advantage that it preserves the angular width of the beam (in combination with additional optics for collimation) as well as the overall efficiency of a device. However, the disadvantage of these mixing rods lies in the fact that good color mixing requires a large length over thickness ratio. Decreasing the thickness of the light-guide can increase the length to thickness ratio. However this will also decrease the in-coupling efficiency and thus the overall efficiency of the device. The alternative is an increase in length of the light-guide. However this is at odds with the object of the device disclosed in WO2006054199 to realize the illumination device in existing lamp form factors. Since the focal point of a parabolic reflector is located near the base of the reflector cup and the out-coupling structure of the light-guide has to be placed in that focal point, the obvious solution for an improvement of the homogeneity would be an extension of the light-guide to the rear of the reflector and into the base. Yet the length of the base is fixed, given the existing form factors.